1. Field of the Invention
The invention relates to easily erased surfaces, such as erasable paper surfaces.
2. Brief Description of the Prior Art
Printed and copied sheet material bearing text or drawings often need correction of the markings for further direct use as copy intermediates, from which new copies can be made. Markings on such sheet materials are physically anchored to the base sheet in different degrees and often cannot be physically removed without damaging the material of the base sheet.
The desirability for easy mechanical erasure without affecting the base sheet has led, in the past, to erasable surfaces obtained through the application of synthetic resin surface layers on the base sheet which prevent the penetration of the markings into the base sheet structure. Such coatings are considered barrier layers. Their adhesion to the base sheet is in general, less than to the markings and as a result, the entire barrier layer is usually removed with the markings during the process of physical erasure.
A shortcoming of such materials is found in that the entire area of erasure around the removed markings-becomes more transparent on opaque sheets and more so on translucent foils or papers where the difference of translucency affects the background uniformity when reprints are made.
We have discovered that barrier coating compositions of our invention consisting of a mixture of alkylketene dimer, hydrocolloids and pigments on base sheets for copying, resist removal by erasing to a greater degree than the prior known barrier layers, while their adherence to conventional marking materials or electrophotographic and even diazo overcoats is reduced. This occurs because the alkylketene dimer component is chemically reactive with the hydroxy radicals of cellulose and hydrocolloids, and chemically bonds to the base sheet. In this way, reproduction materials are obtained wherein the after-applied toners, coatings or markings can be easily removed through physical erasure, without damage to the barrier coating or the base sheet.
Alkylketene dimers have been used in the prior art as both an internal alkaline size and a surface size for paper. The treatment of paper with alkylketene dimer size renders the paper water repellant. The water resistance obtained is stable to acid, alkali or neutral aqueous materials. However, papers sized internally with alkylketene dimers do not exhibit the easy erasability characteristics disclosed in the present invention unless they are additionally coated with compositions of the invention. Surprisingly, it has been found that coating compositions of this invention applied to various paper surfaces not only increase the hold out against water, but also reduce the adhesiveness of various coatings and markings applied thereto afterwards so that they can be easily removed through physical erasure when required. This discovery applies to xerographic markings, to drafting inks, ink jets, pencilling and even diazotype overcoatings.
The base sheets, surface coated with compositions of the invention have been found to be particularly useful as base supports for xerox copying and for drafting applications with inks and pencils.
Erasable materials for xerox copying of the prior art can be considered as base materials with laminated layers which are prepared by applying to a mostly transparent or opaque, paper base, a barrier coating which is impervious to the components of a single or double overcoating with pigments and resins. The barrier coatings consisted mainly of resins in organic solvent systems.
In xerox copying, the toner is applied for imagewise adhesion to the latent picture and the imagewise adhering toner portion is thermally fused and anchored in the barrier coating layer. Handling of the material during the copying steps and afterwards usage for making reprints requires mechanical stability of the material so that no valuable information on the print is deleted.
The above-described prior art processes depend on a precarious balance of the composition of the coating layers.
The barrier layer resins must have an affinity to the toner particles which adhere to the barrier layer by purely adhesive forces before being anchored into the barrier layer by thermal fusion.
An inherent problem of such barrier coating systems is the precarious balance of resin compositions including pigments that finally control toner receptivity and erasability, print contrast, image definition and mechanical print surface resistance.
Higher resin concentrations improve mechanical resistance, but affect negatively erasability.
Higher pigment concentrations improve, ease of erasability, but decrease mechanical print surface resistance.
The present invention overcomes all of the above shortcomings and provides sheets for markings and second originals on opaque and translucent paper, for fine grain reproduction with high reprint contrast and which process easily through conventional electrophotographic copying equipment. They can be easily erased.
A further advantage of the present invention resides in the universal application to xerographic, drafting, diazotype and like processes. Barrier layers, in the past, required specific compositions for the various applications, to fit the particular requirements for diazotypes, xerographic copies or drafting surfaces. They often require substantial coating weights. Such barrier layers often require organic solvents or at least solvent-water mixtures for their applications. The present process enables one to use relatively lightweight aqueous coating preparations in the process of the invention.